Semiconductor piezoresistive transducers and displacement sensors have been widely known and are used in a great variety of applications including those applications having extremely harsh environments. Such devices may employ semiconductor or metal strain gages depending on the environment and application. For such applications, sensors have to be protected from the environment. For many applications the sensors must be contained within a cavity (usually a vacuum) to protect the strain gages. This vacuum cavity is hermetically sealed to maintain the vacuum and protect the sensing elements. Such protected sensors may be employed in displacement pressure sensors and used in many applications including, for example, medical, automotive, and aerospace applications.
Techniques for hermetically sealing semiconductor piezoresistors from hostile environments have generally limited the size of such devices. The reason for this is that additional lateral space is required to accommodate the hermetically sealing cover member. The piezoresistive transducer employs silicon resistive elements whose resistance varies according to intensity or magnitude of applied displacement upon an associated diaphragm. These resistors must be hermetically isolated from the external environment to ensure proper sensing performance and avoid destruction in harsh conditions.
For an example of hermetically sealed environmentally protected transducers, reference is made to U.S. Pat. No. 5,002,901 entitled “Method of Making Integral Transducer Structures Employing High Conductivity Surface Features”, issued on Mar. 26, 1991 to A. D. Kurtz, et al and assigned to the assignee herein. In this patent the piezoresistive elements are formed over the central region of a dielectric layer which overlays a silicon diaphragm. The elements are arranged to form a Wheatstone bridge where four circuit nodes of the bridge are configured as four p+ silicon electrical contact posts disposed on the peripheral corners of the device. Electrical interconnections also comprised of p+ silicon interconnect the contact posts with the piezoresistive transducer elements. A bias voltage is brought to the two contacts where the voltage is measured between the other two contacts. In this manner the hermetic seal for this device is provided by fabricating the peripheral flange on the device's outer periphery beyond the contact posts, and an absolute cavity can be made which provides a vacuum reference. A glass sheet cover is then bonded to the top of the flange to create the hermetic seal. U.S. Pat. No. 5,891,751 entitled “Hermetically Sealed Transducers and Methods for Producing the Same”, issued on Apr. 6, 1999 to A. D. Kurtz, et al and assigned to Kulite Semiconductor Products, Inc., the assignee herein, teaches a hermetically sealed semiconductor transducer and methods for fabricating the same. In this patent a sealing member hermetically seals an aperture whereby a vacuum is maintained between the transducer element and the cover member. The transducer element is hermetically sealed from the external environment while at least a portion of the electrical contact remains exposed to enable subsequent wire bonding thereto.
Reference is also made to U.S. Pat. No. 5,461,001 entitled “Methods for Making Semiconductor Structures Having Environmentally Isolated Elements”, issued on Oct. 24, 1995 to A. D. Kurtz, et al and assigned to the assignee herein. This patent shows a method of fabricating semiconductor structures where one can provide a great number of hermetically sealed individual circuit devices using the methods described in the above noted patent. Reference is also made to U.S. Pat. No. 6,229,427 entitled “Covered Sealed Pressure Transducers and Methods for Making the Same”, issued on May 8, 2001 to A. D. Kurtz, et al and assigned to the assignee herein. That patent shows a method which can be utilized to hermetically seal a raised feature of a sensing network of a silicon on oxide pressure transducer. The invention described in the '427 patent can also be utilized to hermetically seal the depressed feature sensing network of a diffused pressure transducer.
U.S. Pat. No. 5,286,671 entitled “Fusion Bonding Technique for Use in Fabricating Semi Conductor Devices”, issued to A. D. Kurtz, et al on February 1994, relates to silicon oxide pressure transducers and methods for fabricating and bonding to such devices.
In view of the above it is extremely desirable to produce a hermetically sealed displacement sensor, which is easy to fabricate and which provides improved operation over prior art devices.